a. What typed of configuration or technology playing a role ?
Your router seems to be connected to the ISP router and also a wireless router using two interfaces.
ISP created two separate tunnel with failover to wireless from your end to the ISP.

b. How must have the ISP setup ?

ISP created the tunnel interface possibly to route the public IP they provided to you. They have created a static route for the public IP block towards your router.

c) ISP has provided the public ip address and how its routing to ISP.

The public subnet ISP provided to you is being routed via the tunnel interface. As there are multiple hops between your lan and ISP office, they have created the tunnel interface to make it reachable in a single hop. And at ISPs end, they have created a static route pointing towards you for the public IP subnet.

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Internet Assigned Numbers Authority is primarily responsible for assignment of addresses. Users are assigned IP addresses by Internet service providers (ISPs). ISPs obtain allocations of IP addresses from a local Internet registry (LIR) or National Internet Registry (NIR), or from their appropriate Regional Internet Registry (RIR). Source

Now ISPs all public block of addresses are declared in the BGP routing table and share on the internet routing table so that any router on the internet knows which router to go to reach that IP subnet.

ISPs usually provides private RFC 1918 to the customers. At the customer router, there is a default route pointed towards the isp. and at the ISP end, a static route is pointed towards customers so the return traffic can find its way to the customers. Default route at the customer end forwards all traffic towards the ISP and static route at ISP forwards all traffic destined to that specific subnet towards the customer.

Now, when a customer requires a public IP block and ISP allocates that to customer, one way to achieve this is to use one of the public IP configured at the ISP router interface and use that as default gateway address for the customer and another IP on that subnet at the customer router interface and do the NAT to translate internal subnets to the internet. Also do necessary port forwarding to use any of the remaining public IP to be mapped to internal servers.

Here the ISP have provided the customer with two private block of IPs (192.168.73.56/30 & 192.168.70.60/30) and pointed two static routes for these to IP subnets so the return traffic can come back to the customer router. The static route at the ISP might look like below
Ip route 192.168.73.56 255.255.255.252 192.168.73.58
Ip route 192.168.70.60 255.255.255.252 192.168.70.62
And the customer has default routes like we see
ip route 0.0.0.0 0.0.0.0 192.168.70.61 track 1
ip route 0.0.0.0 0.0.0.0 192.168.73.57 100 track 2

Now for the public block of IP the ISP gave to the customer say a.b.c.1/30. ISP will have this IP subnet (or the summary address) declared within their BGP. For the customer, they will create an additional static route pointing to them like below.
Ip route a.b.c.1 255.255.255.255 192.168.73.58 10
Ip route a.b.c.1 255.255.255.255 192.168.70.62 20
To the customer router, this public IPs doesnt need any additional configuration for routing, because there is a default route pointing to the ISP anyway.

Sorry for the delay. That was awesome explanation. Just few more questions

Lets I have internet connection from the ISP called as DIA. I have been given fiber connectivity to the customer router with a block 1.1.1.0/30. 1.1.1.1 will be ISP end and 1.1.1.2 will be customer end

And default route on customer router pointing to ISP router. What that type of connection could be ? Is is point to point connection ?

If you are directly connected to the ISP, its a point to point connectivity. There can be two separate scenarios.

One:
I have a contract with a single ISP to provide Internet connectivity to 8 sites in different cities. The ISP doesn't work in two of the city. What they did is took a lease internet connectivity from one of the local ISPs. They then created a point to point tunnel from their router interface in my remote office and created a tunnel to their office. and routed all my traffic through the tunnel. When I ping my ISP DNS, its not one hop away, although its multiple hop away via second local ISP link

One:
You are connecting to the ISP with a 1.1.1.0/30 subnet. Now you want a public IP say 2.2.2.2 from the ISP for port forwarding requirement to one of your internal servers. Now, ISP can reconfigure the link between you and ISP to 2.2.2.0/30 subnet so you get the 2.2.2.2 in your WAN interface. Or they can create a static route on their router to forward all traffic destined to 2.2.2.2 to your router leaving the current configuration as they are. This is because your subnet connecting to ISP is different to the public IP you have from ISP.

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